This invention relates to a method of producing molded resin products having thick-wall parts.
According to the so-called counter-pressure gas method, which is an example of injection molding method for producing a resin product having thick-wall parts, a gas (such as air or nitrogen gas) with pressure increased beyond the atmospheric pressure is preliminarily injected into a cavity between mold pieces before molten resin is injected thereinto. When a molten resin material having an organic solvent (such as alcohol), an inorganic liquid (such as water), an organic gas such as (Cl.sub.3 H), an inorganic gas (such as N.sub.2, CO.sub.2 and CO) or their mixture dissolved therein as a foaming agent is subsequently injected into the cavity, the foaming gas is thereby prevented from beginning to foam or expand. Only after the preliminarily injected high-pressure gas is removed from the cavity and the pressure inside the cavity is sufficiently reduced, the foaming gas begins to foam and expand inside the product being molded, thereby preventing shrinkage on its surfaces.
The so-called gas assisted injection method is another molding method whereby a cavity in a mold is fully or partially filled with a resin material by primary injection and thereafter, or while it is being injected, a gas is injected into the product being molded.
With the counter-pressure gas method, as described above, the occurrence of shrinkage is intended to be prevented by the force of foaming inside the produce being molded. Thus, the shrinkage force in skin layers on the surfaces cannot be overcome by the force of foaming unless the foaming layer is sufficiently thick (as shown in FIG. 6). In other words, products to be formed by this method are under a severe limitation regarding their shape and wall thickness. In order to obtain a molded product with a smooth surface and a foam layer inside, the wall thickness indicated by arrows in FIG. 7 must be over 5-6 mm.
By the gas assisted injection method, by contrast, it is intended to prevent the occurrence of shrinkage by the pressure of the injected gas. Without the use of any foaming agent, thick-wall parts of a product are not cooled quickly and the high-pressure gas does not effectively form any cavity inside the product being molded, as shown in FIG. 8. Instead, the gas is spread throughout the molded product, affecting adversely the strength of the product. In the case of a product having a thick rib on a thin-wall part, such as shown in FIG. 9, the thin-wall part cools quickly and the gas cannot diffuse to such a part in time, making a hollow cavity only in the rib part. Accordingly, the thickness of the wall parts should be at least about 5 mm.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to eliminate the problems of prior art technology described above.
It is more specifically an object of this invention to a method of producing an injection-molded product having both thick-wall and thin-wall parts and fine foam cells and hollow parts formed in the thick-wall part such that occurrence of surface defects such as shrinkage is prevented.
An injection-molded product to be produced according to this invention, may be characterized as having hollow parts where shrinkage would otherwise be likely to occur such as its thick-wall parts, as well as foam cells between its surface layers and these hollow parts and around the hollow parts. Throughout herein, the expression "foam cells" will be used to indicate small bubbles formed inside a molded resin product by the foaming of a forming gas during its production, and the expression "hollow parts" will be used to indicate larger cavities formed inside a molded resin product by injecting a high-pressure gas.
Before molten resin is poured into the cavity in an injection molding apparatus to produce such a product, a foaming gas is already contained in the resin material and a gas with pressure greater than the atmospheric pressure is supplied into the cavity prior to the injection of the resin material. During or after the injection of the resin material into the cavity, another high-pressure gas at pressure greater than the atmospheric pressure is injected at selected positions inside the product being molded, thereby creating hollow parts. These high-pressure gases are kept inside the cavity for a specified length of time so as to prevent the foaming gas from beginning to foam or expand prematurely. As these high-pressure gases are discharged from the cavity as well as the interior of the product being molded, the foaming gas contained in the molten resin starts to foam and expand such that foam cells are formed between the surface layers and the hollow parts, as well as around the hollow parts, of the product.